Ultra high frequency power measurement



U.H.F. PowEEV MEASUREMENT Filed March 27. 1943 A, z5 ai FEE] J y u 1' 11a I l 14 :5,47 l z? I :-15 15 15 E iz zall` `Dr/,l/V

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BY AUMYYWL ATTORNEY l Patented Apr. 16, 1946 2,398,606 ULTRA HIGH FREQUENCY y POWER MEASURELIEN lChao C. Wang, Montclair, N. J., assignor to Westinghouse Electric Corporation, East Pittsburg Pa., a corporation of Pennsylvania Application 27, 1943, Serial No. 480,883

(Cl. Til-95) 9 Claims.

This invention relates to U, H. F. power measurement and more particularly to power measurement based on the principle veloped by a thermo couple.

A common practice employed prior to the present invention has been to measure power of ultra high frequency, and especially micro-Wave energy', generated by a source of such energy, by absorbing the energy in a volume of water flowing' oi voltage del througha chamber for the purpose. By measuring certain factors, the power can be computed from the factors ascertained or known. The general procedure has been to keep a. steady flow of water running to and from the chamber and to assume the specific heat and water density are constants. Designating the heat dissipated per unit time in the water of the chamber as W, the in-flow temperature of the water as T1, and the out-flow temperature T2 and designating the flow in units of volume velocity by V, the specific heat constant as C and the density of the water as p, the general formula expressing the inherent relationship is: W(T2-T1)VpC'. But it will be observed that all of these factors involve probable errors of measurement, as the absolute value of each is difilcult to ascertain or varies from time to time, and thus the resultant measurement can never be made very reliable under the prior art practice. As calibration of the th'ermo couple depends upon determination of temperature diffel-ences, it will be obvious a, double error is possible and accuracy thereby diminished. The potential evolved is of very small order, and this necessitates use of an exceedingly delicate and expensive millivolt meter. The volume-velocity of the water or other fluid medium employed is measured by a cylinder graduated in length and time visually with 'a stop-watch or the like which again injects possibility of double error. This will be more apparent when remembering that `the surface tension or capillarity of the water concaves its upper surface and that waterdrops will adhere to the glass cylinder where the water is being measured. Timing by a watch can be accurate only to a ilfth of a. second, thereby necessitating at least half a minute time to make even a reasonably accurate reading.

Again', as the property of hydrant water vused will vary from day to day due to temperature variation, purity andl other factors, errors are introduced in assuming specific heat C and the density C of the water as constants. Further error is introduced by heat loss of th'e water through the tubing. Other disadvantages exist,

of whichmay be mentioned 'the nuisanceof keep- A ing the same rate of ow during the time of taking the reading, the difElculty vof reading instantaneous power dissipation, and complications of controlling and watching several instrumentalities simultaneously.

Finally, with respect to the prior art practice it must be borne in mind that avnal probable error is considered mathematically as the square root of th'e sum of the squares ci the individual probable errors, and that the prior art power measuring apparatus, as indicated above, involves numerous individual probable errors the sum of the squares oi which becomes quite large.

The present invention provides a structure y overcoming the above 'disadvantages and probable errors, as its primary desideratum.

A general purpose of the invention may accordingly be stated as 'directed to more accurate and less cumbersome measurement of microwave power.

y More specically, an object of the invention is to eliminate as far as possible the human element of control and reading.

Another object 0f the linvention is to enable accurate instantaneous readings to be made.

A further object of the invention isto enable power readings to be made directly in watts, without need of calculationirom other readings and factors.

Yet another object of the invention isto avoid necessity of and large errors introduced by reading of the minute millivolt potential produced by vthe thermal couple.

In the accompanying drawing -wherein is disclosed a preferred embodiment of the invention, Y

the ligure illustrating the same is a structural sectional and schematic view of genera-l assembly of an ultra high frequency measuring apparatus and system in accordance with my inventive concept.

Referring to said drawing, reference numeral l Ill designates a wave guide of definite cross-section, and usually rectangular, within which wave energy is promulgated in desired mode longitudinally thereof. rThe wave guide I0, for purposes of measuring the power of wave energy, isI terminated by a chamber i I of equal cross-sectional area to the body oi' the wave guide. The electric energy or power to be measured, and indicated in the Wave guide as an EM wave, recognized in theart as designating electro-magnetic wave energy, moves in the wave guide toward the said chamber. 'I'his chamber is divided from the body of the wave guide by a water-tight wallv or partition I2 transparent to the wave energy. Glass h'as been indicated as one material suitable for the purpose. This represents one typical arrangement. 'I'he invention is applicable, however, to'

any other U. H. F. power measurement apparatus in which energy to be measured is dissipated in an enclosed water chamber.

End wall I3 for the chamber Il is shown with an inlet opening I4 near its top and an outlet opening I5 near its bottom. An inlet pipe I6 is in direct communication with inlet opening I4 to supply a how of wat'e'r or other suitable fluid l1 to the chamber. An' outlet pipe Il is in dl'rect communication with outlet opening I5 for the each chamber Il and Il' at substantially equal 'eilluent Iromthe chamber. Water or other fluid is supplied from a main supply pipe I9 to the inlet pipe It. the ow being under control oi' a suitable valve 2l in the inlet pipe.

.A thermocouple is provided within said citiamber il, it being illustrated as providing one electro-thermal element 2| next the inlet opening Il and another electro-thermal element 22 v next the outlet opening I 5.' These two electrothermal elements are electrically insulated from `the fluid and are 4connected in electrical series by appropriate insulated wiring 23 which passes to.

the exterior of the chamber. In practical construction these are thermopiles with junctions located in proximity to the entrance of pipe I1 and I8 respectively. The distance between the junction and the opening of chamber is 'kept small. The wave energy promulgated in the wave temperature produces a voltage difference in the system to a supply reservoir feeding both the' electro-thermal elements, which, in the prior art was measured on a delicate millivolt meter connected across the wires 23 outside the chamber. This minute potential and delicacy of reading made accuracy o f result most dimcult in the prior art.

According to the present invention, I provide a twin chamber Il' having size, shape, and capacity duplicating chamber Il above described. This twin chamber I I has opposite'end walls i2' and Il' otlr material corresponding to like walls for the wave-guide chamber and ,otherwise constructed to be subject tov heat radiation preferably precisely equal to that of the wave-guide chamber. Inlet and outlet 'openings il' andll' next the top andl bottom oi end wall Il' are provided and have a size and relative location the same as in the wave-guide chamber. Outlet pipe il from the wave-guide chamber leads to and constitutes inlet connection for inlet opening Il' and an outlet pipe connection Il' -is connected at and leads from outlet opening il'. 'I'he same'uid and identical volume and velocity ow is therefore obtained in both chambers. A valve 24 may be provided on the outlet pipe il' if desired, to prevent the chambers from emptying when shutting down the apparatus. Ii' the ywave guide II is designed such that the heat lost or gained from surrounding or from the connected wave guide temperatures.

25 within which is a flow of water from the same source supplying water to the chambers. As shown, a branch pipe 2B from main supply pipe I9 enters said tank 25 near its top and a discharge pipe 2l is shown leading from the bottom ofr the tank. Valves 28, 29 in this branch pipe 2l and discharge pipe 21 control the flow and discharge of the water to and fromthe tank. Heat transfer from the water inthe connecting pipe- IB between chambers will thus be governed by the same temperature of /water supplying that pipe by being surrounded by water from the same source. This is only necessary when the temperature difference developed across the thermocouple is large and thermo E. M. F. becomes a non-linear function of temperature diilference. By proper choice of thermo-couple material used and rate of iiow, this cooling system can be eliminated. y

While I have shown the outlet pipe Il' oi' the twin chamber and the cooling tank discharge pipe 21 opening into a funnel drain 30, it is to be understood this is illustrative only, and it is within the scope oi the invention to utilize a closed circuit of water supply, in which event said discharge pipes would .be part of the water-return chambers and coolingA tank. Furthermore the cooling tank shown is to be understood as ex.

emplary of any desired cooling system for equalizing temperature at the twin lchamber inlet Il' to the temperature at the wave-guide chamber inlet i l..

Twin chamber I l' is provided with a heater 3i next partition or end wall I2' for introducing heat in that chamber. The heat is electrically insulated from the water and wallsof the chamber in any approved manner, and is preferably activated'by an electric current under control of a suitable resistance or rheostat 32. A watt-meter 33 is included in the circuit 34 between the rheostat and heater :or obtaining direct reading oi' power input applied to the heater.

Within twin chamber Il' is a thermocou-ple duplicatingA the 4 thermocouple in wave-guide.

This second thermocouple accord- 1 chamber Il. ingly is constituted by one thermopile or electrothermal element 2|' next the inlet opening Il and another thermopile or electro-thermal element 22 next the outlet opening Il'. These two thermopiles or electro-thermal elements are elect'ricallyinsulated from the fluid and are connected in series so that the difference in temperadiii'erences produced in the"respective ther-mov is negligible, then this twin chamber i I' can be designed inmost convenient shape with lnelllslble heat interchange to the surroundings, and thuscouples in said chambers will be equal. The wiring between the two thermocouples of the two chambers is such that potentials produced will e be opposed to each other, and thus when the In o rder to accomplish this ,cie` y sideratum, pipe I l passes through a cooling tank escasos potentials are equal, one, will cancel or nullifythe other. This is a desirable and specinc case of ratio of the potentials in two identical thermocouples requiring no correction factor. A nullindicator is placed in the circuit between the two thermocouples to indicate the degree of nullication. In case the two pairs of thermocouples are not exactly identical, then a calibration for a vcorrection factor can be made, that the null point indicates a predetermined deilriite ratio of power dissipated in chambers Il and Ii.

In operation, power of ultra high4 frequency wave energy in the wave guide ill enters the wave-guide chamber Il and is transformed to heat, thereby raising the temperature of the water of uid I1 in that chamber, and, by Virtue of the temperature diiference at inlet Il and outlet l5 thereof establishes a potential difference in the thermocouple circuit 23. 'Ihis potential difference tends to swing the null indicator 35 from zero position. The operator applies a heating current through rheostat 32, watt meter 33 and circuit 34 to heat the water I1 in the twin chamber Il until potential difference in the thermocouple in that chamber has the desired ratio with the potential difference of the wave-guide thermocouple. 'Ihis ratio, which is preferably equality of potentials, is visualized by swing of the null indicator 35 back to the null or zero position which, in the specific case of identity of the thermocouples gives the ratio of potentials as unity without a, correction factor. Regulation of the current to the heater to maintain the null indicator at the z ero position is then the simple concem of the operator, and while that condition prevails a reading from the watt meter Il gives direct reading oi' power supplied to the twin chamber I I' and that will be the reading for power functioning in the wave-guide i0 since both, in the preferred case. are producing identical and nullifying potentials in their respective thermocouples. The arrangement entirely eliminates any assumptions as to specific heat or density of the water and avoids all necessity for reading of time, heat loss or ilow volume. The wattage is of suillciently high value that the possible error of reading is negligible and thus the measurement of power is accurately. speedily and readily obtained. v

, While the apparatus illustrated is exemplary of one embodiment and in some respects diagrammatic. it is to be understood that appropriate substitutions and refinements of structure may be incorporated without departing from the spirit or scope of the invention either as described heretofore or as claimed hereinafter.

I-claim: 1. Power measuring apparatusv comprising pair of iiuid chambers, means applying electric power sought to be -measured to the fluid of one chamber, means applyinga known electric power to the fluid of the other chamber, the power to the respective chambers being converted to heat in the tiuid of the said chambers, means for indicating state of equality of heat developed in the fluid of the two chambers, and means for vary'- ing the known power to establish equality of heat developed in the two chambers.

2. Power measuring apparatus comprising a pair of fluidr chambers, means applying electric power sought to be measured to the iiuid of one chamber, means applying a second electric power to the fluid of the second chamber, the power to the respective chambers -being converted to heat in the iiuid of the said chambers, means for indicating state of equality or definiteI ratio of heat developed in the iiuid of the two chambers, means for varying the power converted to heat in the second chamber and establishing equality or denite ratio of heat developed in the two chambers. and means for directly indicating the value of the second power as the equivalent of power sought to be measured at the condition of equality of heat developed in the two said chambers.

3. Power measuring apparatus for ultra high frequency electro-magnetic Wave energy comprising a wave guide terminating in a fluid chamber, a second fluid chamber having electrical heating means, a variable source of electrical heating energy and meter for said electrical heating.

means, a connection between said chambers for passage of fluid from one to the other, means for equalizing temperature of fluid entering each chamber, and means for indicating effects of temperature differences between iniiowing and outflowing fluid for both chambers.

4. Power measuring apparatus for' ultra high frequency electro-magnetic wave `energy comprising a wave-guide terminating in a fluid chamber, a second fluid chamber of equal dimensions to the wave-guide chamber, each said chamber having a iiuid inlet and a nuidoutlet, a now connection between the outlet oi' one chamber to the inlet of the other, means for equalizing the temperature of fluid entering at the inlet of each chamber,l a thermocouple in each chamber and each having an electro-thermal element next an inlet 'and an electro-thermal element next an outlet whereby each thermocouple produces a potential, a null indicator, means connecting one thermocouple to the null indicator in opposition to the other thermocouple whereby the null indicator will indicate equal or definite ratio of and opposite potentials in the thermocouples, means for heating the fluid oi. the second iluid chamber to equality or definite ratio with heating oi' the fluid 'in the wave-guide chamber and thereby cause the null indicator to give a zero reading, and means for measuringpower applied to heat the fluid of the second chamber to the aforementioned equality or definite ratio indicated by the null indicator.

5; A method of measuring ultra high frequency electro-magnetic wave energy comprising heating a flow volume of fluid by said energy, heating a like flow volume of fluid by a second source of electric power, indicating the state of equality ordeilnite ratio of heating of both flow volumes, and measuring the second source of power.

6. 'A method oi' measuring ultra high frequency electro-magnetic wave energy `comprising heating a flow volume ofiluid between an inlet and an outlet by the said wave energy, cooling the outtaining equal flow ot fluid in both chambers, l

means for indicating ratio of temperature difi'erence or equality of temperature between inflow and outflow of fluid in the two chambers, means for equalizing change of temperature of the fluid in one chamber to that of the other chamber, and means for measuring heating energy applied in effecting equalization ot they temperatures of the fluid in the two chambers.

8. Electric power measuring apparatus comprising a pair of iluid chambers.. means for maintaining equal iiow of fluid in both chambers.

means for equalizing inflowtemperature of iluid to both chambers, means for indicating ratio of temperature diierence or equality of temperature between inilow and outflow of fluid in the two chambers, means for equalizing lchange of temperature oi' the iiuid in one chamber to change of temperature of the iiuid in the other chamber, and means for measuring heating energy applied in effecting equalization oi' the temperatures of -the uuid in the two chambers.

9. Electric power measuring apparatus compr'ising a pairfof iluid chambers, means for apseance plying electical energyv to be measured 'tothe fluid in one of said chambers, means for maintaining equal ilow of iluld in both chambers, a thermocouple in each chamber, a null indica.- tor wired to both thermocouples for indicating temperature change between nilow and 'outflow oi iluid in the two chambers, means for applying electrical heatingenergy to the iluid in the second one of said chambers for equalizing temperature change of the uid therein between inilow and outflow to the temperature change in the iluid in the irst said one ,of said chambers,

and an electrical meter for measuring the applied electrical heating energy to the second one of said chambers in eilecting the temperature equalization. v

CHAO C. WANG.: 

